System for detecting radio-frequency identification tags

ABSTRACT

Techniques for detecting radio-frequency identification (RFID) tags are disclosed. For example, an exit control system is described that detects unauthorized removal of articles from a protected facility. A series of antennas are setup to produce interrogation corridors located near the exit of the protected area. RFID tags are attached to the articles to be protected. Each tag includes information that uniquely identifies the article to which it is affixed and status information as to whether the articles removal from the facility is authorized. The RF reader outputs RF signals through the antennas to create electromagnetic fields within the interrogation corridors. The reader outputs RF power from a single port to the multiple antennas via a splitter/combiner. In this way, a single RF reader with only one transmitter/receiver port simultaneously interrogates multiple antennas. A variety of techniques are described by which the reader can detect the removal of an unauthorized article.

TECHNICAL FIELD

The invention relates to the use of radio frequency identificationsystems for management of articles within a protected area and, morespecifically, to techniques for detecting unauthorized removal ofarticles from a protected area.

BACKGROUND

Radio-Frequency Identification (RFID) technology has become widely usedin virtually every industry, including transportation, manufacturing,waste management, postal tracking, airline baggage reconciliation, andhighway toll management. RFID systems are often used to preventunauthorized removal of articles from a protected area, such as alibrary or retail store.

An RFID system often includes an interrogation zone or corridor locatednear the exit of a protected area for detection of RFID tags attached tothe articles to be protected. Each tag usually includes information thatuniquely identifies the article to which it is affixed. The article maybe a book, a manufactured item, a vehicle, an animal or individual, orvirtually any other tangible article. Additional data as required by theparticular application may also be provided for the article.

To detect a tag, the RF reader outputs RF signals through the antenna tocreate an electromagnetic field within the interrogation corridor. Thefield activates tags within the corridor. In turn, the tags produce acharacteristic response. In particular, once activated, the tagscommunicate using a pre-defined protocol, allowing the RFID reader toreceive the identifying information from one or more tags in thecorridor. If the communication indicates that removal of an article hasnot been authorized, the RFID system initiates some appropriate securityaction, such as sounding an audible alarm, locking an exit gate, and thelike.

Most methods of determining whether articles present in theinterrogation corridor have been checked out depend upon firstindividually detecting and identifying each tag in the field, and thenchecking determining the status of the articles associated with theidentified tags. Some methods, for example, involve determining a serialnumber for each tag, and then accessing a database to determine thestatus of the article associated with the identified serial number.Other techniques require issuing commands to the identified tags oncethe serial number has been determined.

This process can be time-consuming, especially if several tags exist inthe field. For example, in order to obtain a complete tag serial number,only one tag can respond at a time. If more than one tag responds at atime, a collision occurs, the data received may be invalid, and neithertag's serial number can be obtained. To deal with this, some systems usean anti-collision process, which requires each tag to respond in adifferent time slot until all tags are heard. This added delay isundesirable in an exit control system because patrons are in theinterrogation corridor for a very short period of time. Also, eachpatron can be carrying multiple books. The time required to determinewhether every one of the books is checked-out is often much longer thanthe time a patron spends in the corridor.

SUMMARY

In general, the invention relates to a Radio-Frequency Identification(RFID) system for detecting radio-frequency identification tags. Morespecifically, the invention relates to an RF exit control system whichdetects unauthorized removal of articles from a protected facility, suchas books or other articles from a library. A series of antennas are setup to produce interrogation corridors located near the exit of theprotected area. RFID tags are attached to the articles to be protected.In one example system, each tag includes information that uniquelyidentifies the article to which it is affixed and status information asto whether the article is authorized to be removed from the facility. Todetect a tag, the RF reader outputs RF signals through the antennas tocreate an electromagnetic field within the interrogation corridor. An RFreader outputs RF power from a single port to multiple antennas via asplitter/combiner. In this way, a single RF reader with only onetransmitter/receiver port simultaneously interrogates multiple antennas.The field activates the tags, and the tags, in turn, produce acharacteristic response. The RF reader receives the tag information viathe single transmitter/receiver port and the RF exit control systemdetermines whether removal of the article is authorized. If removal ofthe article is not authorized, the exit control system initiates someappropriate security action, such as sounding an audible alarm, lockingan exit gate, etc.

In one embodiment of the invention, an exit control system includes aplurality of radio frequency (RF) antennas set up to provide one or moreinterrogation corridors and a RF reader coupled to the plurality ofantennas, the RF reader having a transmitter/receiver (T/R) port thatprovides each of the antennas with RF power to produce interrogationfields within the interrogation corridor. The system may further includea splitter that receives the RF power from the RF reader and deliversthe RF power to each of the plurality of antennas in the form of aplurality of antenna drive signals.

In another embodiment, a method comprises producing a radio frequency(RF) output signal from a single transmitter/receiver (T/R) port of anRF reader, splitting the RF output signal into a plurality of antennadrive signals, and delivering the antenna drive signals to a pluralityof antennas to produce interrogation fields within one or moreinterrogation corridors. The method may further include generating oneor more input signals with the antennas in response to at least one tagpresent within the interrogation fields, combining the input signalsinto a combined input signal, and providing the combined input signal tothe T/R port of the RF reader.

In another embodiment, a computer-readable medium comprises instructionsthat cause a processor to receive from a single reader a tag detectionsignal that indicates at least one tag is present within any of aplurality of interrogation corridors, receive a patron signal thatindicates at least one patron is present within any of the interrogationcorridors, and output an alarm signal upon receiving the tag detectionsignal and the patron signal within a time period. The computer-readablemedium may further comprise instructions to cause the processor toinitiate a timer upon receiving either of the tag detection signal orthe patron signal and output the alarm signal upon receiving the otherone of the tag detection signal or the patron signal prior to expirationof the timer.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a radio frequency identification(RFID) system for management of articles traveling into and out of aprotected area.

FIG. 2 is a more detailed block diagram of the RF exit control system.

FIG. 3 is a graph showing the drive field signals for each of theantennas in a three-antenna RF exit control system.

FIG. 4 is a block diagram that illustrates controller in further detail.

FIG. 5 is a flow chart illustrating the overall operation of the RF exitcontrol system.

FIG. 6 shows the frame format for communication between an RF reader andRF tags.

FIG. 7 shows two example tag signals.

FIG. 8 shows a flowchart of one embodiment of a method employed by theRF reader to determine presence of a checked-in tag in the interrogationcorridor.

FIG. 9 shows an example tag signal in the presence of noise.

FIG. 10 shows another embodiment of a method employed by the RF readerto determine presence of a checked-in tag in the interrogation corridor.

FIGS. 11A and 11B show alternate embodiments of the signal strengthindicator algorithm.

FIG. 12 shows another embodiment of a method employed by the RF readerto determine presence of a checked-in tag in the interrogation corridor.

FIG. 13 shows another embodiment of a method employed by the RF readerto determine presence of a checked-in tag in the interrogation corridor.

FIG. 14 shows embodiment of a method employed by the RF reader todetermine presence of a checked-in tag in the interrogation corridor.

DETAILED DESCRIPTION

In general, techniques are described herein for detectingRadio-Frequency Identification (RFID) tags. More specifically, thisdescription is directed to techniques that utilize an RF exit controlsystem to detect unauthorized removal of articles from a protected area.The protected area is generally of the type in which the removal ofarticles must be authorized, such as books in a library or items in aretail store. Each article in the facility contains an RFID tag, whichmay uniquely identify the article to which it is affixed. In addition,for purposes of the present description, the RFID tag also containsstatus information indicating whether removal of the article isauthorized. The RFID tag may be embedded within the article so that thetag is substantially imperceptible, to help prevent tampering. An exitcontrol system determines if removal of the article from the facilityhas been authorized (e.g., a book has been properly checked-out by alibrary patron or staff member) and sets off an alarm if it has not.

FIG. 1 is a block diagram illustrating a radio frequency identification(RFID) system 10. Exit control system 5 detects unauthorized removal ofarticles from a protected area 7. For purposes of the presentdescription, the protected area will be assumed to be a library and thearticles will be assumed to be books or other articles to be checkedout. Although the system will be described with respect to detectingchecked-in tags to prevent their unauthorized removal from a facility,it shall be understood that the present invention is not limited in thisrespect, and that the techniques described herein are not dependent uponthe particular application in which the RFID system is used. Forexample, the system could also be used to check for other kinds ofstatus or type information without departing from the scope of thepresent invention.

Exit control system 5 includes lattices 9A and 9B which define aninterrogation zone or corridor located near the exit of protected area7. The lattices 9A and 9B include antennas for interrogating the RFIDtags as they pass through the corridor to determine whether removal ofthe item to which the tag is attached is authorized. As described infurther detail below, exit control system 5 utilizes a single reader todrive multiple antennas. To detect a tag, an RF reader outputs RF powerthrough the antennas to create an electromagnetic field within theinterrogation corridor. The RF reader outputs RF power from a singleport to multiple antennas via a splitter/combiner. In this way, a singleRF reader with only one transmitter/receiver port simultaneouslyinterrogates the corridor using multiple antennas. The field activatesthe tags and the tags, in turn, produce a characteristic response. TheRF reader receives the tag information via the singletransmitter/receiver port and the exit control system determines whetherremoval of the article is authorized. If removal of the article is notauthorized, the exit control system initiates some appropriate securityaction, such as sounding an audible alarm, locking an exit gate, etc.

In addition, the overall RFID system 10 may include a number of “smartstorage areas” 12 within protected area 7. For example, an open shelf12A, a smart cart 12C, a desktop reader 12E and other areas. Each smartstorage area 12 includes tag interrogation capability which enablestracking of articles throughout a facility. In a library setting, forexample, a book could be tracked after check-in while en route to ashelf 12A on a smart cart 12C.

The RFID tags themselves may take any number of forms without departingfrom the scope of the present invention. Examples of commerciallyavailable RFID tags include 3M™ RFID tags available from 3M Company, St.Paul, Minn., or “Tag-it” RFID transponders available from TexasInstruments, Dallas, Tex. An RFID tag typically includes an integratedcircuit operatively connected to an antenna that receives RF energy froma source and backscatters RF energy in a manner well known in the art.The backscattered RF energy provides a signal that may be received by aninterrogator within RFID system 10 to obtain information about the RFIDtag, and its associated article.

An article management system 14 provides a centralized database of thetag information for each article in the facility. Article managementsystem 14 may be networked or otherwise coupled to one or more computersso that individuals, such as a librarian, at various locations, canaccess data relative to those items. For example, a user may request thelocation and status of a particular article, such as a book. Articlemanagement system 14 may retrieve the article information from adatabase, and report to the user the last location at which the articlewas located within one of the smart storage areas. Optionally, thesystem can re-poll or otherwise re-acquire the current location of thearticle to verify that the article is in the location indicated in thedatabase.

FIG. 2 shows a more detailed block diagram of an example embodiment ofthe RFID exit control system 5. As illustrated, exit control system 5 isconfigured for transmitting and/or receiving data from one port of RFreader 20 to/from multiple antennas according to the techniquesdescribed herein.

More specifically, exit control system 5 includes antennas 8A, 8B and 8C(collectively referred to as “antennas 8”) positioned to providemultiple interrogation zones 40A and 40B. Each antenna 8A–C includes anassociated tuner 18A–C through which the antennas are connected to RFreader 20 and ultimately to controller 14. Although FIG. 2 shows system10 as including three antennas 8A–8C and two interrogation zones 40A and40B, it shall be understood that exit control system 5 can include anynumber of antennas set to provide any number of interrogation zonesdepending upon the needs of the facility.

Exit control system 5 operates within a frequency range of theelectromagnetic spectrum, such as 13.56 MHz, with an allowable frequencyvariance of +/−7 kHz, which is often used for Industrial, Scientific andMedical (ISM) applications. However, other frequencies may be used forRFID applications, and the invention is not so limited.

Antennas 8 may be designed to develop electromagnetic fields of at leastcertain strengths within the interrogation corridors 40. This may beadvantageous for one or more reasons, including improving the likelihoodof detecting tags having the desired status, e.g., tags that arechecked-in in a library application. In one embodiment, theelectromagnetic fields created by the antennas 8 are used to power theRF tags in the corridors 40. The amount of energy induced in each RF tagis proportional to the strength of the magnetic field passing throughthe tag loop. The antennas 8 therefore may produce a field having amagnitude that exceeds a threshold magnitude for energizing an RF tag,such as 115 dBuA/m. In addition, the magnitude preferably meets orexceeds the threshold magnitude throughout a substantial volume of theinterrogation corridor. For example, the field produced may have amagnitude that exceeds the threshold magnitude for 50%, 75%, 90%, 99%,or more of the volume of the interrogation corridor, thus increasing thelikelihood that unauthorized (i.e., tags that are still checked-in) RFtags in the corridor are successfully detected.

The RF Reader 20 of exit control system 5 may also read/write datafrom/to the RFID tags. RF reader 20 outputs RF power from onetransmitter/receiver port 21 to multiple antennas 8 via asplitter/combiner 42. In this way, one RF reader 20 with only onetransmitter/receiver port 21 can simultaneously use multiple antennas tointerrogate RF tags. In the embodiment shown in FIG. 2, thesplitter/combiner 42 is external to RF reader 20 such that the system iseasily scalable. Thus, to accommodate a different number ofinterrogating antennas, only the splitter/combiner 42 need be changed.

RF reader 20 receives a response from the RFID tags through the samesplitter/combiner 42 and transmitter/receiver port 21. The receivedsignal is analyzed by the system to determine whether a checked-in(e.g., not checked out) article is present in an interrogation corridor40.

By providing RF power to each antenna with RF reader 20, each antenna 8receives RF power and none of the antennas 8 need rely onelectromagnetic coupling to a driven antenna to get power. This greatlyimproves the detection capability of the exit control system 5 underconditions where electromagnetic coupling is inadequate, such as whenthe antennas are not large enough or close enough together to allowefficient coupling.

Because RF reader 20 receives a response from the RFID tags through thesame splitter/combiner 42, the return signal from any RF tags in thecorridor are combined going back through the splitter/combiner 42 intothe RF reader transmitter/receiver port 21. In this way, if a weak tagsignal is received by antenna 8A and a weak signal for the same tag isalso received by antenna 8B, for example, the two weak signals fromantennas 8A and 8B are combined at splitter/combiner 42. The combinedsignal is then input into RF reader 20 through transmitter/receiver port21. This greatly increases the likelihood detecting even weak tagsignals.

In an example embodiment, the exit control system 5 detects only whetherat least one checked-in tag is present in the corridor. There areseveral situations in which numerous tags could be present in thecorridor. For example, one patron could be carrying multiple articlesthrough the corridor. Alternatively, multiple patrons, each carrying atleast one article, could pass through the same or different corridorssimultaneously. Furthermore, because of the relatively short period oftime it takes for a patron to pass through the corridor, there typicallyis not enough time to receive and analyze individual information foreach and every tag that may be in the corridor. By combining theindividual signals from each of the antennas in the system, the signalreceived by the RF reader will indicate simply whether at least onechecked-in tag is present in the corridor. The present exit controlsystem is thus designed such that even when numerous tags are present inthe corridor, if at least one of them has checked-in status, the systemwill alarm. Similarly, when numerous tags are present in the corridorand more than one of them has checked-in status, the system will alarm.The librarian or other designated employee can then check the articlesto determine which of the articles present when the system alarmed havenot been properly checked-out. The methods by which the system maydetermine presence of a checked-in tag (i.e., one that has not beenproperly checked-out and is therefore not authorized to be removed fromthe facility) are described in further detail below with respect toFIGS. 7–14. Although the system is described with respect to detectingpresence of checked-in tags to prevent their unauthorized removal from afacility, it shall be understood that the present invention is notlimited in this respect. For example, the system could also be used tocheck for other kinds of status or type information without departingfrom the scope of the present invention.

Photocells 24A and 24B, one for each interrogation corridor 40A and 40B,respectively, signal presence of a patron in their respective corridors.Interconnects 16A, 16B and 16C connect the alarms 12 and photocells 24to controller 14. A counter 22 may also be included which incrementseach time one of photocells 24 detect a patron in the corridor.

In one embodiment, each antenna 8 nominally receives the same amount ofRF power from RF reader 20, but, as will be described in more detailbelow, is driven ninety degrees out of phase with its neighboringantennas. The phase shift, by creating a rotating field betweenantennas, enhances the ability of the system to detect tags regardlessof the orientation of the tag. In this manner, the exit control system 5transmits and receives from one RF reader transmitter/receiver port 21to multiple antennas 8 via a splitter/combiner 42. Antennas 8 receivenominally the same amount of power from the RF reader, but are driven90° out of phase with each other.

In one embodiment in which a response is received for a checked-in tag,the RF reader 20 communicates with the controller 14, which may enablealarms 12. In FIG. 2, alarms 12 include visual alarms 12A and 12C andaudible alarm 12B, although any combination of visual, audible, or othermethod of communicating checked-in RF tag presence may be used.

FIG. 3 is a graph that shows the resulting phase shift of the RF drivesignals 43A–C for each antenna 8A–8C (FIG. 2), respectively. As shown inFIG. 3, the RF drive signal 43B for antenna 8B is 90° out of phase withthe RF drive signal 43A for antenna 8A; the RF drive signal 43C forantenna 8C is 180° out of phase with antenna 8A, etc.

The phase shift allows the system to detect RF tags in all orientationsby creating a rotating field between the antennas. Thus, regardless ofthe orientation of the RF tag as it travels through the interrogationcorridor, the likelihood of detection is increased.

Various methods can be used to achieve the 90° phase shift betweenneighboring antennas. In one embodiment, the antennas are connectedusing transmission lines that differ by ¼ wavelength between neighboringantennas to achieve the desired 90° phase shift. For example, referringagain to FIG. 2, lines 32A, 32B and 32C which connect the antennas 8A,8B and 8C to splitter/combiner 42 could be implemented by couplinglengths of ¼ wave transmission lines as appropriate to drive eachsuccessive antenna 90° out of phase as shown in FIG. 3.

In another embodiment, compensation circuitry could be provided at eachantenna 8A–C to adjust the phase shift induced by transmission lines32A–C such that the resulting phase shifts are 90° out of phase as shownin FIG. 3.

The exit control system 5 thus provides several advantages. One RFreader with only one transmit/receive port can be used to simultaneouslyutilize multiple antennas. By providing RF power to each antenna at acontrolled amplitude and phase, magnetic coupling is not relied upon todeliver power to the antennas and to control the relative phase of eachantenna. Also, because the interrogating fields are driven to produce arotating interrogation field, coverage is increased in the interrogationcorridor. In addition, the system is scalable—namely, the number ofinterrogating antennas to be utilized in any particular system can beaccommodated by changing only the RF splitter. Weak signals frommultiple antennas are combined to form an adequate signal, thus alsoincreasing the likelihood of detecting signals. Moreover, EM emissionsare reduced by driving the antennas at a 90° phase shift, as the farfields for any antennas driven 180° apart will cancel.

FIG. 4 is a block diagram that illustrates controller 14 in furtherdetail. As illustrated, in the embodiment depicted in FIG. 2, controller14 receives an input signal 45 from interconnect 16A that indicates apatron has been detected in corridors 40. In addition, controller 14receives an input signal 47 from RF reader 20 that indicates that the RFreader has detected at least one signal within corridors 40. In anembodiment, as described in further detail below, controller 14continually monitors input signals 45 and 47. When input signals 45 and47 indicate that both a patron and a checked-in tag have been detected,controller 14 initiates an alarm.

FIG. 5 is a flowchart 50 further illustrating exemplary operation ofcontroller 14. As illustrated, controller 14 performs a continuous loopmonitoring that looks for a checked-in tag in the corridor, or for apatron to enter the corridor, and initiates an alarm only when both apatron and a checked-in RF tag are detected in an interrogationcorridor. Thus, controller 14 continually monitors input signals 45 and47 to determine whether a checked-in RF tag (52) or a patron (54) ispresent in any of corridors 40A or 40B. If either one of theseconditions is met, controller 14 starts a timer (56 or 58,respectively). The purpose of the timer is to ensure presence of both apatron and a checked-in RF tag in the corridor at essentially the sametime, for example, within 0.5 seconds, or some other time as may beappropriate.

Controller 14 next determines whether the other criteria, namely eithera patron (60) or a checked-in RF tag (62) is present. If not, controller14 checks whether the timer has timed out (64 or 66, respectively). Ifso, then both a patron and a checked-in RF tag were not present withinthe allotted time frame, and controller 14 returns to the beginning ofthe loop. In the event that both a patron and a checked-in tag arepresent in the corridor within the allotted time frame, controller 14activates the alarm (68).

Various techniques by which the exit control system determines whetheran unauthorized tag is present in the corridor will now be described. Inone embodiment, the techniques described herein allow RF reader 20 toquickly determine whether any articles that are not properly checked-out(in other words, articles that have checked-in status and are thereforenot authorized to be removed from the facility) are in the interrogationcorridor. The techniques allow RF reader 20 to rapidly and accuratelydetermine the presence of articles with checked-in status in thecorridor, and will minimizes the adverse impact of tag collisions thatmay otherwise degrade the system performance.

As described above, quickly determining the presence of a tag withchecked-in status can be important because of the relatively shortperiod of time in which each patron is in the interrogation corridor,and the fact that multiple patrons can be in the interrogation corridorat the same time. The present techniques described below enable this inseveral ways. First, RF reader 20 does not necessarily require receiptof a full tag serial number for each tag in the corridor in order todetermine the status of the tag. For example, in some embodiments, allof the checked-in tags in the corridor may respond at the same time. Inother words, the techniques do not necessarily require that each tag inthe corridor respond in a separate time slot so that each tag can beindividually identified. In fact, in some embodiments, there is no needto even individually identify each tag in the corridor to determine thestatus of the tags. In some embodiments described below, thetransmission of a complete, single communication frame is not required.

RF reader 20 and the RF tags communicate using a known protocol in whicheach message is embedded within one or more frames of a predefinedformat. The format of an example RFID transmission frame 100 is shown inFIG. 6. The frame 100 includes a start of frame (SOF) 102, a message104, cyclical redundancy check (CRC) 106 and end of file (EOF) 108. SOF102 indicates the beginning of the frame. Similarly, EOF 108 indicatesthat the entire frame has been transmitted. Any non-fixed data isembedded in the message 104 portion of the frame 100 and CRC 106reflects the data in the message 104.

CRC 106 is used to check the integrity of the data. To calculate CRC106, all bits of the data are pushed through a predetermined algorithm.Once the frame is transmitted, the receiver decodes CRC 106 using thereceived data to determine whether the message 104 was properlytransmitted. If the CRC generated from the received data does not matchthe CRC contained within the frame itself, an error occurred.

One aspect of the presently described techniques is directed to ensuringthat only tags that are not checked-out (i.e., still checked-in) respondwhen passing through the interrogation corridor. This can beaccomplished by making use of a feature called the Application FamilyIdentifier (AFI) byte. This feature is described in the ISO 15693standard for RFID systems. The AFI byte is a piece of memory in the RFIDtag that contains one 8-bit value. The AFI is normally used to identifythe type of article to which the tag is attached, such as book, CD,videotape, etc. The value stored in the AFI location can be changedthrough a defined series of commands described in the ISO 15693standard. When the RF reader issues an AFI command it sends an AFIvalue. As defined in the ISO 15693 standard, when the AFI valuetransmitted in a command is 0×00 (hexadecimal) then all tags in theinterrogation field respond. When the RF reader transmits any valueother than 0×00, then only tags with a matching AFI value in memoryrespond to the command.

The techniques described herein use the AFI byte to indicate the statusof the article, for example, whether the article has been checked-out.The AFI field is therefore used as a checked-in/checked-out status byte.When books or other articles are on the shelf the AFI byte is set to adesignated “checked-in” value. When a librarian checks out the book or apatron checks out at a self check station the AFI value is changed to adifferent, “checked-out” value.

The RF reader scans for tags containing the checked-in value in theirAFI memory location. This will cause all tags with their AFI byte set to“checked-in” to respond. If the RF reader receives a response from thetags then the item was not properly checked-out. This is because anyitem that was properly checked-out would not have the checked-in valuein their AFI byte and would not respond.

An example of how the present technique of using the AFI byte as achecked-in/checked-out status byte will now be described. A patronreturns an item to an automatic book drop. The book drop reads theserial number and sets the AFI byte to “checked-in”. The item isreturned to the shelf and then another patron decides to leave with theitem. The new patron inadvertently leaves without checking-out the book.The patron walks through the interrogation corridor, which is lookingfor tags having the “checked-in” value. When the system sees thechecked-in tag in the corridor, the system will alarm.

If instead the patron properly checks-out the item, the AFI byte is setto “checked-out”. When the patron passes through the corridor, the tagwill not respond to the system's command because the system asks onlychecked-in tags to respond. The patron can thus walk through thecorridor and remove the article without alarm.

A second technique described herein is directed at verifying that thereceived tag communication is actually a tag-produced response and notnoise-produced. Namely, in one embodiment, the system asks all tags inthe interrogation field to respond at the same time. Under normalcircumstances this would only be done in a situation where one tag waspresent in the field at a time. When two or more tags respond in thesame timeslot it creates a situation called a “collision”. Normally whena collision occurs, no message 104 of the responding tags can be heardproperly. In many systems, a process called anti-collision isimplemented and tags are commanded to respond in different timeslotsuntil all tags are identified. However, this process typically consumestoo much time in exit control applications, in which tags pass quicklythrough the corridor.

Instead, the techniques described herein ask all tags to respond in thesame time slot knowing that collisions will occur if more than oneunchecked-out (checked-in) tag is present in the corridor. Thisembodiment makes use of the fact that the SOF is one piece ofinformation that can still be validly received even when collisionsoccur. The SOF is the first transmission sent by tags responding to acommand. Regardless of how many tags respond to the command they willall respond with the same SOF at the same time. By detecting the SOF,the system validates that at least one checked-in tag is actually in theinterrogation field.

FIG. 7 shows an example of two tag signals, a first tag signal 110 and asecond tag signal 112. Both signals 110 and 112 transmit the same SOF,but have different data in the message fields. Since both tags areresponding at the same time and the data is combined by thesplitter/combiner going back into RF reader, the data in the messagefield received by the RF reader is subject to a collision. The SOF,however, does not collide irrespective of how many tags are in thefield.

FIG. 8 is a flowchart 130 of the present technique for verifyingpresence of an unchecked tag in an interrogation corridor using the AFIbyte as a status byte and the SOF as verification that an unchecked tagis in the field. First, the RF reader sends the AFI command with the AFIvalue set to checked-in (134). Each tag with a matching checked-in AFIbyte responds, and the possible checked-in tag response is received(136). Next, to verify that the response is an actual tag response andwas not created by noise, the system checks for the SOF (138). If avalid SOF was received, at least one checked-in tag is present in theinterrogation corridor (140) and the alarm is activated (142). The alarmis activated for a predetermined duration. On the other hand, if a validSOF is not received, the system assumes that noise caused the response,and that therefore there is not a checked-in tag in the corridor (146).The loop then restarts by sending an AFI command (134).

In other embodiments techniques to ensure the validity of the SOF areused. In particular, a received signal strength indicator is used toseparate actual tag-produced responses from noise-produced responses.FIG. 9 shows an example of tag signal frame in the presence of noise.The tag signal 114 is shown on top of a noise floor 120. This noisefloor is measured and analyzed as described below to verify a valid SOF.

FIG. 10 is a flowchart 170 showing this technique. FIG. 10 is similar toFIG. 8 in that the AFI byte is used as a checked-in/checked-out statusbyte and the SOF is used to validate a tag-produced signal. In addition,the flowchart of FIG. 10 uses a received signal strength technique tovalidate the SOF. First, the AFI command is sent with the AFI byte setto checked-in (166). Then, the noise floor of the corridor is measured(164) prior to any tag response. Any tag with AFI byte set to checked-inwill respond to the command, the response is received and the signalstrength is measured (168). The system next checks for the SOF (170). Ifa SOF is detected, the response signal strength is compared with thenoise floor (172). This is because although receipt of an SOF is anindication that a tag is in the field, since the SOF is only 8-bits longnoise can sometimes produce an SOF sequence. If the differential betweenthe response signal strength and the noise floor is adequate to indicatethat the signal is authentic (174), the system validates that achecked-in tag is present in the interrogation field (176). The systemthen alarms (178). On the other hand, if the differential is notadequate (174), the system assumes that noise created the response andthat therefore there is no checked-in tag in the corridor (182). Theloop is then restarted.

FIGS. 11A and 11B show two embodiments of the method for comparing thepossible received tag response with the noise floor (172 in FIG. 10). InFIG. 11A, method 172A first looks for a difference between the measurednoise floor and the measured signal strength, the signal strengthmeasurement occurring during the time of the signal response period,that is adequate to indicate that the received signal is authentic(202). If the signal strength differential is adequate to indicate thatthe signal is authentic (204), the RF reader indicates that anunchecked-out (i.e., checked-in) tag is present in the corridor (208).If the signal strength differential is not adequate to indicate that thereceived signal is authentic, the RF reader assumes that noise producedthe response and that therefore no checked-in tags are present in thecorridor (206).

In FIG. 11B, the method not only looks at the noise floor before theSOF, but also after the end of the expected tag response. Checking thenoise floor after the EOF is received is one more verification that theresponse was really a tag-produced response and not a noise-producedresponse. The method 172B first looks for a differential between thenoise floor measured prior to the SOF and the signal strength (220). Ifthe differential is not adequate to indicate that the signal isauthentic (222), the system assumes a noise-produced response andsignals that no checked-in tags are present in the corridor (226). Ifthe differential is adequate, (222), the system next looks for adifferential between the signal strength and the noise floor measuredafter the EOF of the expected tag response (228). If this differentialis also adequate (230), the system signals that a checked-in tag ispresent in the corridor (232).

The techniques described in FIGS. 10 and 11 may have several advantages.By having all the tags respond in the same time slot, the amount of timerequired to determine whether a checked-in tag is present in theinterrogation field is significantly reduced. The minimum scan time isreduced from around 60 ms to around 20 ms. In addition, when all thechecked-in tags respond at the same time, the likelihood of detecting achecked-in tag is increased because the signals are combined going backinto the RF receiver. Also, since the system only looks for the SOF, thewhole tag transmission need not be heard to determine whether or not itis checked-in. This can occur when a tag moves into a weaker portion ofthe interrogation field and loses power half way through thetransmission. In this way, the system can reliably alarm even if a tagonly has enough power to transmit part of its serial number. In fact,the checked-in tag need only transmit its SOF for the system to detectits presence. Furthermore, the system is not compromised when multipletags respond at the same time. In fact, the system is designed so thatthis is the case. Multiple tag responses occurring at the same timeactually increase the likelihood that a checked-in tag will be detected.

The signal strength indicator can be implemented using a variety ofembodiments. In one embodiment, the signal strength indicator isgenerated by a circuit, and provides an indication of the strength ofthe received signal. This information is amplified and sent to thecontroller (reference numeral 14 in FIG. 2). The controller 14 uses ananalog-to-digital converter to analyze the signal as described abovewith respect to FIGS. 11A and 11B.

FIG. 12 shows another embodiment of a method by which the RF reader maydetermine whether a checked-in tag is present in the interrogationcorridor. This process (250) is used with the “Tag-it” type tagsavailable from Texas Instruments as mentioned above. There is a commandin the Tag-it protocol where all tags in the interrogation field respondwith the data stored in an defined block and they will all respond atthe same time. The present technique sets one block of data in the tagas the “check-out status block.” The command is then used to determinewhether at least one unchecked-out (checked-in) tag is in the field.

For example, assume the check-out status block in a checked out book isset to:

00000001

and the data in a checked-in book is set to:

00000000.

As tags move through the interrogation field, the “read unaddressedblock” command is sent by the RF reader. Every tag in the field willrespond at the same time. As the tags respond the RF reader will receivethe SOF as described above. The check-out status block for each tag willbe identical except for the last bit and the CRC if both checked-out andchecked-in tags are present. The present method checks for collisions onthe last bit of the check-out status block and the CRC to determinewhether at least one checked-in tag is present in the interrogationfield. For example, the following table shows the possibilities that mayoccur, where “clear” indicates no collision was detected.

All checked-out books SOF - clear No alarm Checked-out status - clearCRC - clear All checked-in books SOF - clear Alarm Checked-out status -clear CRC - clear Both checked-out and SOF - clear Alarm Checked-inbooks Checked-out status - collision on last bit CRC - collisions

In reference to FIG. 12, the RF reader sends the “read unaddressedblock” command (252). The possible tag response is received (254). Thesystem checks for SOF using the techniques described above with respectto FIGS. 8, 10 and/or 11. If the SOF is not detected (256), the readercontinues checking (252). If an SOF was detected, the system checks fora collision on the last bit of the checked-out status byte (258). If acollision is detected (260) the reader next checks the CRC (262). If acollision is detected in the CRC the system signals that at least onechecked-in tag is in the corridor (264) and activates the alarm (266).This is the situation shown in row three of the Table discussed above.

If no collision occurred in the checked-out status bit (260) the readerdetermines whether the checked-out status bit is set to checked-in(268). If so, the reader checks for collisions in the CRC (270). Ifthere are no collisions, then all of the tags in the corridor arechecked-in tags (272) and the controller activates the alarm (266). Thisis the situation shown in row two of the Table shown above.

If no collision occurred in the checked-out status bit (260) and thechecked out status bit is not checked-in, then the books must bechecked-out, there are no checked-in tags in the corridor (274) and thenext response is checked (276). This is the situation shown in row oneof the Table shown above.

Other embodiments may also be used to determine presence of a checked-intag in the interrogation corridor. One of these embodiments is shown inFIG. 13. The flowchart may depict an algorithm that runs continuously inthe RF reader. When the RF reader indicates an alarm (320) a message issent to the controller notifying that a checked-in book has passedthrough the interrogation corridor. This algorithm shown in FIG. 13ignores collisions and overpowered tags during the first sweep for tags.The algorithm concentrates on reading as many tags as possible asquickly as possible.

The algorithm shown in FIG. 13 allows the current TI “Tag-it” and TI ISO15693-3 tags to be used in an exit control system without significantdegradation of performance. One conventional way to determine whetherthe TI-type tags are checked-in is to run a SID (SimultaneousIDentification) Poll on all tags in the field and then read from thespecific block where the checked-in/checked-out code is kept. Onepotential problem with this technique is that the SID Poll will continueuntil all collisions are resolved (when multiple tags talk at the sametime) and if one tag leaves the field during this algorithm, then theprocess halts and no data is returned. This could very easily happen ina detection environment where tags are constantly moving into andleaving the field—patrons walking through the exit control system withbooks (which have tags).

The algorithm shown in FIG. 13 uses a modified SID poll (302) and triesto resolve as many tags as possible as quickly as possible the firsttime through the algorithm (304 and 310). If a tag with a set checked-incode is found (312), the alarm is triggered (320). If there is stilltime remaining (304) (the tags are still in the field), then thealgorithm will attempt to resolve as many collisions as possible (308).Because it only issues the unmasked SID poll and ignores collisionsunless there is enough time to resolve them, the speed with which thealgorithm can identify unchecked-out tags in the field is increased.

The statistics for this method are described below. The first set ofnumbers given in parentheses are examples of least significant digits ofthe SID code. This is based on the 16 timeslot SID algorithm. The secondset of numbers in parenthesis is the number of tags validated after thefirst pass.

-   0 Tags 100.00%-   1 Tag 100.00%-   2 Tags: 15/16 chance of reading 93.75%-   3 Tags: No Collisions (012) 210/256 chance (82.03%) One    Collision (011) 45/256 chance (17.57%) Two Collisions (111) 1/256    chance (0.39%)    Overall Chance:-   No Collisions+⅓*One Collision+0*Two Collisions 87.89%-   4 Tags: No Collisions (0123) 2730/4096 chance (66.65%) One    Collision (0012) 1260/4096 chance (30.76%) Two Collisions (0001)    60/4096 chance (1.46%) Double Collision (0011) 45/4096 chance    (1.10%) Three Collisions (0000) 1/4096 chance (0.02%)    Overall Chance:-   No+½ One+¼ Two+0 Double+0 Three 82.31%-   5 Tags: No Collisions (01234) 32760/65536 (49.99%) One    Collision (00123) 27300/65536 (41.66%) Two Collisions (00012)    2145/65536 (3.27%) Three Collisions (00001) 1290/65536 (1.97%) Four    Collisions (00000) 1/65536 (0.00%) Two/Two (00122) 1890/65536    (2.88%) Two/Three (00111) 150/65536 (0.22%)    Overall Chance:-   No+⅗ One+⅖ Two+⅕ Three+⅕2-2 77.26%

It shall be noted that with 3 tags, if there are no collisions, all tagswill be verified. If there is one collision, the colliding tags will notbe read, but the one tag which does not match will be read, and sincethis is one of the three possible tags, a factor of ⅓ is used tomultiply by the percentage chance of having one collision. This logic iscontinued throughout the other 4 and 5 tag statistics. It shall also beunderstood that these percentages are only for the first pass, and theother tags will be resolved on subsequent passes, time and location ofthe tags permitting.

Another embodiment of the checked-in tag detection is shown in FIG. 14.The algorithm refers to a database containing tag ID's of properlychecked out items. This database that may reside on the exit controlsystem itself, but the invention is not limited in this way.

First, an SIF poll is performed on the tags in the interrogationcorridor (352). When tags are detected in the interrogation fields(354), the algorithm only gathers the tag ID's that can be resolvedbefore the tags leave the interrogation corridor (360). The amount oftime before a tag leaves the interrogation corridor could either bedetermined as an average calculated value based upon expected speedthrough the portal or be dynamically determined by the inventory of tagsbeing collected.

An additional poll of tags could also be required after each collisionis resolved. If this poll doesn't get a least one duplicate tag ID asthat was obtained in the initial poll, the determination could be that anew set of tags has entered the interrogation corridor. This wouldtrigger the database query for the previous set of tags.

Another possible strategy would be to infer that the tag has left thecorridor as soon as the current collision cannot be resolved. This alsowould trigger the database query.

The algorithm shown in FIG. 14 thus specifically focuses on collectingas many tag ID's as possible and does not check for security informationuntil the above mentioned time is expired. At that time, a query is madeto the database to determine if all the detected tags exist in thesecurity database.

The advantages provided by the embodiment of FIG. 14 are related to theeffects of non-uniform fields, as well as the amount of time availablefor tag detection. This algorithm provides a means of maximizing thenumber of tags to sample for security. This algorithm is not affected assignificantly by non-uniform fields because once the tag ID iscollected, the system no longer needs to communicate with the tag forsecurity information.

The following discussion is directed toward a method for use with thenew Electronic Product Code (EPC). The EPC is set to supplant theUniversal Product Code (UPC) in certain applications by using RFID foritem identification. Within this new specification is a “destroy”command that when executed renders the RFID tag destroyed ornonfunctional. The method creates a “key” for this destroy command whichis difficult to detect as well as secure so that malicious use of thedestroy command will not affect performance of the RFID tag.

The destroy command renders the RFID tag nonfunctional. To set thedestroy code, a proper command is given to the chip and the memory isprogrammed. To execute the destroy command, the password which wasplaced in the destroy memory location must be sent again to the chip,and if there is a match then the chip is destroyed.

The present method creates a secure “key” for the destruction of RFIDtags. If one key was used for all tags at all locations, if someone wereto break the key, they could in theory destroy all RFID at thatlocation. For example:

Tag A Tag B Tag C Destroy Code: G G G

The same code in the destroy register yields a possibility ofcompromising all tags in an installation.

With the present method, however, the EPC identification code (up to 88bits of information) is run through an algorithm and further placed intothe destroy memory register (24 bits). This would make every destroycommand unique to each tag, and would make a unique key that isdifficult to decipher. For example:

Tag A Tag B Tag C Destroy Code: U W L

An algorithm “key” is common to all tags and destroy codes, but becausethe destroy code cannot be read from the tags, the presently describedmethod makes it much more difficult to break the algorithm, thusmaintaining the overall security of the site.

Example EPC Identification: 00000000 00 hex 11111111 FF 00000000 0011111111 FF 00000000 00 11111111 FF 00000000 00 11111111 FF 00000000 0011111111 FF 00000000 0088 bits organized into 11 blocks of 8 bits.

An example algorithm may, for example, select some memory, perform afunction (add, subtract, multiply, with data or constants, etc.) andcreate an output destroy command.

Destroy command 10001101 8D (random for this example) 01111011 7B00010110 16

Another EPC value with this algorithm run would create an entirelydifferent destroy command value.

Between sites, a different algorithm could be used to discern differentstores or vendors such that the different algorithm would not allow thetags to be destroyed. In a shipping security example, only articles soldto one retailer to could be sold by that same retailer. Between twostores with different algorithms, the identical EPC value would yield adifferent destroy command code.

Various embodiments of the invention have been described. These andother embodiments are within the scope of the following claims.

1. A system, comprising: a plurality of radio frequency (RF) antennasset up to provide one or more interrogation corridors; a RF readercoupled to the plurality of antennas, the RF reader having a singletransmitter/receiver (T/R) port that provides each of the antennas withRF power to produce interrogation fields within the interrogationcorridors and delivers a combined input signal to the RF reader, whereinthe RF reader generates a tag detection signal to indicate that at leastone tag is present within the interrogation corridors; a splitter thatreceives the RF power from the RF reader and delivers the RF power toeach of the plurality of antennas in the form of a plurality of antennadrive signals, wherein the splitter receives one or more input signalsfrom the plurality of antennas and combines the one or more tag signalsto form the combined input signal; a plurality of sensors to detect apatron within any of the interrogation corridors and generate a patronsignal; and a controller that outputs an alarm signal upon receiving thetag detection signal and the patron signal within a time period.
 2. Thesystem of claim 1, wherein the plurality of antennas generate the inputsignals in response to at least one tag present within the interrogationfields.
 3. The system of claim 1 wherein the splitter combines the inputsignals such that a weak input signal from one of the antennas iscombined with a weak input signal from at least one other antenna toincrease the likelihood of detecting a tag in the corridor.
 4. Thesystem of claim 1, wherein the interrogation corridors are located nearthe exit of a protected area.
 5. The system of claim 1, wherein thecontroller initiates a timer upon receiving either of the tag detectionsignal or the patron signal, and outputs the alarm signal prior toexpiration of the timer upon receiving the other one of the tagdetection signal or the patron signal.
 6. The system of claim 1 whereineach antenna receives RF power from the reader that is out of phase withits neighboring antennas to produce rotating interrogation fields withinthe interrogation corridor.
 7. A system, comprising: a plurality ofradio frequency (RF) antennas set up to provide one or moreinterrogation corridors; an RF reader coupled to the plurality ofantennas, the RF reader having a single transmitter/receiver (T/R) portthat provides each of the antennas with RF power to produceinterrogation fields within the interrogation corridors and delivers acombined input signal to the RF reader; and a splitter that receives theRF power from the RF reader and delivers the RF power to each of theplurality of antennas in the form of a plurality of antenna drivesignals, wherein the splitter receives one or more input signals fromthe plurality of antennas and combines the one or more tag signals toform the combined input signal, wherein the RF power delivered to eachof the antennas has a 90° phase difference from the RF power deliveredto a neighboring one of the antennas, and wherein the 90° phasedifference is provided using ¼ wavelength transmission lines.
 8. Thesystem of claim 1 wherein the T/R port simultaneously provides each ofthe antennas with the RF power and accepts a signal produced by an RFtag in any of the interrogation corridors.
 9. A method, comprising:producing a radio frequency (RF) output signal from a singletransmitter/receiver (T/R) port of an RF reader; splitting the RF outputsignal using a splitter into a plurality of antenna drive signals;delivering the antenna drive signals to a plurality of antennas toproduce interrogation fields within one or more interrogation corridors;generating one or more input signals with the antennas in response to atleast one tag present within the interrogation fields; combining theinput signals into a combined input signal using the splitter; providingthe combined input signal to the T/R port of the RF reader; outputtingthe tag detection signal from the RF reader to a controller; receiving apatron signal that indicates whether a patron is present within any ofthe interrogation corridors; and outputting the alarm signal uponreceiving the tag detection signal and the patron signal within a timeperiod.
 10. The method of claim 9, further comprising: receiving thecombined input signal with the T/R port; and generating a tag detectionsignal from the combined input signal to indicate that at least one tagis present within the interrogation corridors.
 11. The method of claim10, further comprising producing rotating interrogating fields in theinterrogation corridor.
 12. The method of claim 9, further comprising:initiating a timer upon receiving either of the tag detection signal orthe patron signal; and outputting the alarm signal prior to expirationof the timer upon receiving the other one of the tag detection signal orthe patron signal.
 13. The method of claim 9, further comprisingdelivering the plurality of antenna drive signals to the plurality ofantennas such that adjacent antennas are driven out of phase.
 14. Themethod of claim 9, further comprising delivering the plurality ofantenna drive signals to the plurality of antennas-such that adjacentantennas are driven 90° out of phase.
 15. A method comprising: producinga radio frequency (RF) output signal from a single transmitter/receiver(T/R) port of an RF reader; splitting the RF output signal using asplitter into a plurality of antenna drive signals; delivering theantenna drive signals to a plurality of antennas to produceinterrogation fields within one or more interrogation corridors;generating one or more input signals with the antennas in response to atleast one tag present within the interrogation fields; combining theinput signals into a combined input signal using the splitter; providingthe combined input signal to the T/R port of the RF reader; anddelivering the plurality of antenna drive signals to the plurality ofantennas using ¼ wavelength transmission lines such that adjacentantennas are driven 90° out of phase.
 16. An exit control system fordetecting unauthorized removal of articles from a protected area, theexit control system comprising: a plurality of antennas oriented toprovide interrogation corridors; an RF reader coupled to the pluralityof antennas, the RF reader having a single transmitter/receiver (T/R)port that provides RF power to the antennas to produce interrogationfields in the interrogation corridors and delivers a combined inputsignal to the RF reader, wherein the RF reader interrogates theplurality of antennas using the single T/R port to transmit RF power tothe antennas and to receive tag signals from the antennas at the singleT/R port, wherein the reader generates a tag detection signal toindicate that at least one tag is present within the interrogationcorridors; a splitter that receives the RF power from the RF reader anddelivers the RF power to each of the plurality of antennas in the formof a plurality of antenna drive signals, wherein the splitter receivesone or more input signals from the plurality of antennas and combinesthe one or more tag signals to form the combined input signal; aplurality of sensors to detect a patron within any of the interrogationcorridors and generate a patron signal; and a controller that outputs analarm signal upon receiving the tag detection signal and the patronsignal within a time period.
 17. A computer-readable medium comprisinginstructions that cause a processor to: output RF power from a readerusing a splitter to a plurality of antennas through a singletransmitter/receiver (T/R) port to produce interrogation fields within aplurality of interrogation corridors; receive from the splitter via theT/R port a combined tag detection signal that indicates at least one tagis present within any of the plurality of interrogation corridors;receive a patron signal that indicates at least one patron is presentwithin any of the interrogation corridors; initiate a timer uponreceiving either of the tag detection signal or the patron signal; andoutput an alarm signal upon receiving the tag detection signal and thepatron signal prior to expiration of the timer.